The composition of atherosclerotic plaques in 331 five-mm segments of the 4 major (left main, left anterior descending, left circumflex, and the right) epicardial coronary arteries of 8 patients with juvenile (mean age onset, 9 years; mean age of death, 29 years) diabetes mellitus was determined by computerized planimetric analysis. Analysis of all coronary segments disclosed that the plaques consisted primarily of dense (53%) and cellular (38%) fibrous tissue. Pultaceous debris (7%), foam cells (1.2%), and calcific deposits (0.7%) occupied a small percentage of the plaques. Thus, 91% of the coronary plaques in these young diabetic patients consisted of fibrous tissue and nearly all of the remaining 9% consisted of lipid deposits. Analysis of composition according to degrees of cross- sectional luminal narrowing revealed marked increases in dense fibrous tissue (from 31% to 74%), pultaceous debris (from 3% to 12%), and calcific deposits (from 0% to 3%) as the cross-sectional area narrowing increased from less than/equal to 25% to greater than 75%. Compared to older patients with fatal coronary artery disease, the patients with juvenile diabetes had more dense fibrous tissue and pultaceous debris and less cellular fibrous tissue and calcific deposits.